Hollow core photonic band gap (HC-PBG) fibers have been fabricated from silica glass and reported in the literature. See, e.g., Cregan et al., “Single-mode photonic bad gap guidance of light in air,” Science, 285(5433), 1537-1539 (1999); Barkou et al., “Silica-air photonic crystal fiber design that permits waveguiding by a true photonic bandgap effect.” Optics Letters, 24(1), 46-48 (1999); Venkataraman et al., “Low loss (13 dB/km) air core photonic band-gap fibre,” ECOC, Postdeadline Paper PD1. 1, Sep., 2002; and Venkataraman et al., “Microstructured optical fibers and preforms and methods for fabricating microstructured optical fibers,” U.S. Pat. No. 6,847,771 (2005), the entire contents of each are incorporated herein by reference. FIG. 1 shows a schematic of the cross-section of a HC-PBG fiber. The periodic layered structure of holes and glass creates a photonic band gap that prevents light from propagating in the structured region (analogous to a 2D grating) and so light is confined to the hollow core. The core of the fiber in FIG. 1 takes the place of seven small holes in the center of the periodic structure, thus this arrangement is referred to as a 7-cell HC-PBG fiber. Similarly, a 19-cell HC-PBG fiber has a larger core, which takes the place of 19 small holes in the center of the periodic structure. Typically, the periodicity of the holes is on the scale of the wavelength of light and the outer glass is used for providing mechanical integrity to the fiber. The fact that light travels in the hollow core also means that the losses will be lower so longer path lengths can be used. Also, non-linear effects will be negligible and damage thresholds will be higher so that higher power laser energy can be transmitted through the fiber for military and commercial applications. Additionally, since light is guided in the hollow core, an analyte disposed therein will have maximum interaction with light, unlike traditional evanescent sensors.
The periodicity of the holes, the air fill fraction (defined by the ratio of void volume to solid material volume in the microstructured region, i.e., the region comprising the plurality of holes and solid material therebetween, and exclusive of the core and jacket regions), and the refractive index of the glass dictate the position of the photonic band gap, namely the transmission wavelengths confined to the hollow core and guided within the fiber. HC-PBG fibers are obtained by first making a structured preform and then drawing this into a microstructured fiber with the correct overall dimensions. The preform is typically comprised of a central structured region, which is typically made by stacking common circular tubes, which is inserted into a supportive common circular outer jacket tube. This assembly process inevitably introduces interstitial voids between the stacked tubes, which must be eliminated in the final fiber. These voids are similarly sized to the intended holes in the structured region of the preform, and run the entire length of the preform, therefore making fiberization difficult. This is especially true for specialty oxide and non-oxide glasses where the vapor pressure during fiberization may be sufficient to prevent collapse of these interstitial voids. Furthermore, the accuracy of the periodicity and position of the intended holes is important for bandgap guidance in the fiber, and is adversely affected by incorrect tube positioning and tube slippage during fiberization, which are common deficiencies of the tube stacking method using common circular tubes. Specialty glasses typically have much lower softening temperatures than silica glasses, making them more amiable to other forming processes such as extrusion, wherein tubes with arbitrary outer and inner transverse shapes can be fabricated and used both as stacking elements and as the supportive outer jacket. Silica glasses are not easily adaptable to extrusion, due to their high softening temperatures.